Many patients that suffer from congestive heart failure (CHF) develop a wide QRS complex resulting from a delayed activation of one of the ventricles in the heart. This ventricular “dysynchrony” may be caused by dilation of the heart, which disrupts the conduction pathways and interferes with depolarization sequences. Ventricular dysynchrony may worsen heart failure symptoms.
In a classic case of ventricular dysynchrony, the patient's right ventricle activates first, and the left ventricle activates at a later time. The patient often experiences a reduction in cardiac output because the ventricles begin contraction at slightly different times. The timing imbalance may also cause the patient to experience paradoxical septal motion, mitral regurgitation or decreased ventricular filling time.
Patients having a wide QRS complex may receive benefits from an implanted medical device, such as a pacemaker, that paces both ventricles. The implanted medical device senses atrial contractions, waits a predetermined time (or atrioventricular (AV) delay) after each sensed atrial contraction, and then paces both ventricles. The ventricles may be paced simultaneously, or one ventricle may be paced before another. This bi-ventricular pacing is one form of cardiac resynchronization, and it provides many CHF patients with improvements in quality of life, exercise capacity and overall cardiac function.
Generally speaking, cardiac resynchronization refers to pacing therapies applied by implanted medical devices with pacing leads in two or more chambers of the heart. In response to a sensed event, the pacemaker delivers pacing pulses to two chambers of the heart. The pacing pulses may be, but need not be, delivered simultaneously. Although the discussion that follows emphasizes bi-ventricular pacing to treat ventricular dysynchrony, cardiac resynchronization also encompasses, for example, resynchronization of atrial contractions.
Cardiac resynchronization, particularly bi-ventricular pacing, may increase the workload on the heart. A patient that responds to cardiac resynchronization typically experiences an increase in ejection fraction, i.e., the ratio of the blood ejected from a ventricle to the volume of the ventricle.
In addition, a patient that responds to cardiac resynchronization may suddenly experience cardiac failure at a later time. It is not known whether the cardiac failure is due to increased workload on the heart or to other factors. Although this phenomenon is subject to ongoing research, some cardiologists are concerned that some patients who initially respond well to cardiac resynchronization may later respond poorly.